Steam turbine components made of Ni--Cr--Mo, Ni--Mo--V, Ni--Cr--Mo--V, and Cr--Mo--V alloys, such as rotors and discs, provide optimum high-temperature fatigue and creep properties as well as medium temperature - high cycle fatigue properties, but are considered difficult to weld. However, since the down time associated with the upgrade or replacement of these existing often worn, eroded, or cracked components can cost utilities hundreds of thousands of dollars per day, many weld procedures have been attempted to upgrade or repair them.
One such repair procedure consists of welding an individual piece of forged steel to an existing or worn rotor or disc. However, when this type of repair is made on a single rotor blade groove fastening, herein referred to as a "steeple," welder accessibility is very limited. Accordingly, a weld repair conducted with very limited accessibility can result in unacceptable, nondestructive examination quality due to the formation of porosity cracks, lack of fusion, and slag inclusions.
It is also known to make rotor repairs by submerged arc welding after a low volume welded seam is made between a turbine component and a forged replacement section. These types of processes are discussed in U.S. Pat. Nos. 4,213,025 (Kuhnen) and 4,219,717 (Kuhnen). In such a procedure, a ring forging is welded to a worn disc or rotor, or a completely new rotor forging is welded to replace the entire end of the rotor. For example, U.S. Pat. No. 4,633,554 (Clark et al.), discloses a narrow gap weld root pass followed by a gas metal arc build up for this purpose. The lower tensile and fatigue properties obtained by employing this process, however, are generally insufficient for use in high stress rotor steeple areas.
Submerged arc welding alone has also been used for build up repairs of rotor areas involving a wide or deep groove, where a crack or defect is not obtained longitudinally along the radius of the rotor. The main advantage of building up with submerged arc welding is that this procedure has a very high deposition rate, typically about 15 pounds of weld metal per hour. The higher deposition rate is important since many of the service rotor weld repairs are made during turbine outages. Thus, time is extremely important. However, this procedure requires a preheat and produces a relatively large grain size with inferior metallurgical properties. Typically, these submerged arc welds on low pressure rotors have a yield strength of about 85 to 100 Ksi (586 to 689 MPa) and a room temperature Charpy toughness of about 100 to 120 ft-lbs (136 to 163 J). It is also understood that submerged arc weldments are often rejected due to poor ultrasonic quality, which often reveals slag inclusions and porosity in the weld metal. Moreover, serious creep-rupture and notch-sensitivity problems have been encountered with high-pressure Cr--Mo--V rotor repair welds manufactured from submerged arc weldments. Thus, the submerged arc process is generally unacceptable for weld repairs of Cr--Mo--V rotor steeples having small, high-stress concentration radii.
Gas metal arc procedures have also been employed for repairing rotors and discs. This weld procedure deposits about 8 lbs. of weld metal per hour, typically having slightly better properties than weldments produced by the submerged arc process. For Cr--Mo--V rotor repair welding, the gas metal arc weldments of steel turbine components generally have a yield strength of about 85 to 100 Ksi (586 to 689 MPa), and a room temperature Charpy toughness of about 110 to 130 ft-lbs (150 to 177 J). The gas metal arc welding process associated with welding these alloys, however, is often associated with arc-blow (magnetic) process limitations which can limit the use of this process.
Recently, emphasis has been placed on the use of gas tungsten arc welding processes (GTAW) for making repairs on Ni--Mo--V and Ni--Cr--Mo--V low-pressure rotor components. This emphasis can be seen in R. E. Clark, et al. "Experiences with Weld Repair of Low Pressure Steam Turbine Rotors," 47th American Power Conference, Apr. 22-24, 1985, Chicago, Ill., printed by Westinghouse Electric Corporation, Power Generation, Orlando, Fla. Gas tungsten arc welding has been employed for repairing individual rotor attachment grooves, cosmetic, or shallow groove repairs to correct minor surface defects. It has also been used to allow multiple build-ups of blade or component attachment or groove locations, i.e., for a 360.degree. application, and cladding or build-up to restore worn-away material. Gas tungsten arc welding offers relatively high ultrasonic quality, requires less preheat, and produces weldments having tensile and impact properties which exceed rotor material specification requirements. Low alloy steel weldments produced by this process typically have a yield strength of about 90 to 115 Ksi (621 to 793 MPa), and a room temperature Charpy toughness of about 160 to 210 ft-lbs (218 to 286 J). In addition, this welding procedure produces the finest microstructural grain size of any of the above mentioned processes.
It is also known that the selection of a weld method depends on factors such as distortion, non-destructive testing acceptance limits, and mechanical property response to the post-weld heat treatment. Each area of a turbine rotor is unique, and experiences a different service duty. The absence of weld and heat affected zone cracking as well as the minimization of defects, can only be accomplished by carefully controlling a number of welding variables. For the gas tungsten arc welding process, some of these variables include amperage, alloy selection, joint geometries, and travel rate. The parameters selected should be accommodating to automatic welding processes to obtain a uniform quality which is reproducible from weld to weld. These parameters must also produce superior welding characteristics such as freedom from porosity, cracking, and slag entrapment, while being accommodating to all possible repairs on rotors and discs. Finally, the alloy and welding parameters selected must produce a weld comparable to the properties of the base metal.
Weld repair by controlled weld build-up and re-machining of turbine components, including more failure resistant turbine rotors and methods for repairing worn surfaces of steam turbines, especially high pressure turbine rotors, is known in the art. This type of process is discussed in U.S. Pat. Nos. 4,940,390 (Clark et al.) and 4,903,888 (Clark et al.). In such a procedure, a rectangular shaped weld volume is created from which a plurality of fingers is machined. These methods include welding procedures and heat treatments that minimize weld stresses and cracking. The procedure of a controlled weld build-up substantially reduces the risk of failure in ferrous Cr--Mo--V base metals of high pressure, high temperature rotors and discs commonly found in steam turbines. This procedure provides better welder accessibility and weldment integrity, resulting in an improvement over welding forged fastenings to the rotors. These features are particularly important with respect to high pressure turbine components, such as rotors, which have been known to operate at pressures over 2400 psi and temperatures over 1000.degree. F.
The process of controlled weld build-up consists of depositing a first layer of weld metal on a prepared surface of a turbine component and then depositing a second layer of weld metal over the first layer, using a higher application temperature, for tempering at least a portion of the "heat-affected zone" (HAZ) created in the base metal by the depositing of the first layer. As used herein, the term "heat-affected zone" refers to the area of the base metal immediately adjacent to the fusion zone of the weldment. This process uses improved welding methods for overcoming the occurrence of metallurgical structural problems within the heat-affected zone. The additional heat generated by the deposition of the second layer of weld metal produces an immediate heat treatment of the heat-affected zone, whereby coarse grains in the base metal are recrystallized and tempered. It is understood that when coarse grains are reformulated into a finer grain structure, stress-relief cracking in the vicinity of the weld repair can be minimized. This controlled weld build-up process also avoids the over-tempering, or softening, of the base metal created by the heat of welding the first layer of the weld metal. This loss in strength occurs, to a great extent, when a stress transverse to the weld is applied, for example, high and low fatigue, tensile, or creep-to-rupture. The proper control of the initial layer of weldment can significantly reduce the failure in the heat-affected zone and prevent the loss of strength in the zone below the levels of the unaffected base metal. It is also known to include the use of bead sequencing to minimize the heat input into the base metal. In addition, a weld trail-shield is employed to minimize carbon losses in the weld metal which could result in lower tensile properties. Also, parameters such as preheat-interpass temperatures, shield gas-type and flow rates, current, voltage, tungsten size and travel speed are also known for achieving a higher quality weld. Procedures for single "steeple" repairs and 360.degree. rotor repairs are also known. However, this process is very time consuming in that a large single weld volume must be built up, a single weld bead at a time, using a single torch and then this single weld volume must subsequently be machined to form individual fingers. This down time or cycle time associated with the repair or replacement of turbine rotors and components may be relatively costly. This procedure also wastes material due to the fact that the weld metal is first deposited on the surface to be repaired and then a portion of the single weld volume must be machined off to form the individual fingers.
This invention relates to an improved welding process that improves the metallurgical properties of the upgrade or repair area of the turbine component while at the same time reduces the cycle time for conducting these upgrades or repairs. This improved welding process reduces the repair or cycle time by employing multiple welding torches to form multiple weld volumes or fingers concurrently. Welding individual weld volumes also has the benefit of reducing the amount of weld metal that must be used to effectuate the repair and the amount of machining that is required after welding. This invention thereby reduces the overall repair or cycle time and at the same time provides improved metallurgical properties of the weld.